Over the last fifty years, growth in electrical demand and in generating capacity has far outpaced the construction of transmission lines to carry that power from the point of generation to the point of demand. The shortfall in transmission capacity has been due both to public antipathy to opening of new electric power transmission rights of way and construction of new transmission lines and to changes in incentive structure caused by deregulation of the electric power industry.
In response to the shortage of transmission capacity the industry has (a) developed equipment for better allocating the flow of electricity among alternative transmission paths, (b) basing current ratings on weather conditions prevailing at the time of electrical loading, (c) developed conductors which, for the same size and weight, can be operated at higher current levels and higher temperatures and (d) developed methods for replacing older conductors with those of higher rating and doing so without interrupting operation of the circuit begin upgraded.
The above measures notwithstanding, many of the structures which support today's high voltage transmission lines are well beyond their design life and in need of replacement. Even structures in good condition are mechanically unable to accommodate larger conductors necessary to make major increases in the current-carrying capacity of lines.
The foregoing create a strong incentive for building new, higher capacity transmission lines on existing rights-of-way. New lines of higher voltage, using modern structure designs and consisting of two rather than one circuit may increase the transfer capability of an old right of way by as much as ten to one.
Unfortunately those transmission paths where the need for increased transmission capacity is the greatest, are also the paths where removing the existing line from service to allow construction is difficult or impossible. The only remaining option is to build a new line while the old one remains in service; either permanently or until the old can be replaced by the new.
Rights of way are normally too narrow to allow safe construction of a new transmission line alongside the old while the latter is still operating; hence the need for methods and equipment which can provide space sufficient for safe construction of new structures while an existing line continues to operate. The invention described herein addresses that need.
While those versed in the art will recognize applicability and extension of the invention to other types of transmission line support structures, this description will focus on an “H-frame” structure, typical of many 115 kV, 138 kV and 230 kV transmission lines throughout the world. Such a structure 1 (prior art) is illustrated in FIG. 1 and usually consists of two upright wooden poles 2 which support a cross arm 3 from which three strings of insulators 4 are suspended, one for each of three phase conductors 6 used in conventional high voltage power transmission. The structure is normally reinforced by an X brace 5. A small “shield wire” 7 is normally attached to the top of each pole 2 to intercept lightning strokes.
In FIG. 1 the phase positions are labeled a, b, and c. Individual structures of the type shown in FIG. 1 are separated by a distance which may vary from several hundred to over a thousand feet. The conductor 6, being suspended at each structure 1 forms a catenary between structures, the lowest point of which, shown as a′, b′, and c′ in FIG. 1, will be at the center of the “span” between structure s on perfectly level terrain. In FIG. 1 a and a′, b and b′, as well as c and c′ are linked by a dashed line. FIG. 2 (prior art) shows a longitudinal view of such a span.